In an ordinary small motor, the external surface of its motor casing has a circular shape (round shape) (see FIG. 6(A)). When such a round motor is to be mounted in an apparatus or on a wiring board, a whirl-stop must be employed for preventing rotation of the motor. When the round motor is mounted in an apparatus, the round shape tends to be accompanied by wasted space within the apparatus. Thus, in view of prevention of rotation of a motor in relation to a mounting surface and space efficiency, there is known impartment of a quadrangular or higher polygonal external shape to a motor.
FIG. 8 shows a quadrangular motor having a 4-pole field system modified from a motor having a 6-pole field system described in Patent Document 1. In the illustrated quadrangular motor, the side wall of a motor casing, which serves as a yoke, is formed into a quadrangular shape, and four magnets which serve as four field poles are attached to the respective inner surfaces of side portions of the quadrangular side wall. For the purpose of lowering vibrations of the side portions, the magnets are fixed at the centers of the corresponding side portions. A rotor having six rotor poles is supported rotatably on the interior side of the magnets. Generally, individual magnets are shaped such that the radial clearance between the magnet and the outer circumferential surface of the rotor pole gradually increases from the circumferential center of the magnet toward the circumferentially opposite ends of the magnet so as to gradually weaken the magnetic field intensity, thereby avoiding an abrupt change in the magnetic field intensity, which would otherwise result from rotation of the rotor. Thus, cogging torque can be lowered.
However, in order to lower cogging torque, the individual magnets of FIG. 8 are formed such that the distance between the magnet and the rotor at the circumferentially opposite end portions of the magnet is rendered greater than that at a circumferentially central portion of the magnet. Nonetheless, the circumferentially opposite end portions of the magnet are still thicker than the circumferentially central portion of the magnet, and rendering the distance greater apparently causes deterioration in performance. Furthermore, wasted spaces are formed between the magnets. Thus, disposing the magnets in the respective central regions of the side portions of the yoke raises the following problem: the magnets can be neither reduced in size nor arranged efficiently; consequently, the motor thickness (radial distance between opposed side portions of the quadrangular yoke) is increased, resulting in a failure to reduce the size of the motor.
In order to cope with the above problem, there is known a configuration in which the magnetized portions are disposed at respective corners between side portions of a quadrangular yoke (see Patent Documents 2, 3, and 4). FIG. 9(A) is a sectional view showing a motor which is described in Patent Document 4 and in which the magnetized portions are disposed at the respective corners between the side portions, and FIG. 9(B) is an enlarged view showing one of the four magnetized portions. In FIG. 9(A), the motor casing has a quadrangular section and accommodates therein a 4-pole field magnet magnetized with alternating N and S poles. This field magnet is magnetized such that the centers of magnetic poles are located at the respective corners between the side portions of the yoke. The motor casing has a fixed plate thickness and is shaped such that its corners are crushed inward while the side portions of the quadrangular section are partially left intact; and the magnet is shaped such that its inside has a circular surface and such that its outside is in close contact with a motor frame.
As shown in FIG. 9(B), this magnet is intended to minimize the magnet weight while motor characteristics are retained, by means of optimizing the degree of inward crush of the corners; i.e., the ratio of a dimension L2 of a thickest portion of the magnet to a dimension L1 of a thinnest portion of the magnet, or the ratio of a radius R1 of a corner curvature to an inside radius R2 of the magnet.
In order to improve productivity, desirably, the field magnet of a small motor is manufactured separately from the motor casing (yoke) and is then assembled with the yoke. Usually, such assembly employs a press-fit technique. However, for press-fitting, a relative dimensional tolerance between the motor casing and the magnet must be set strictly. However, since the motor configuration as shown in FIG. 9(A) is designed to crush the corner portions of the quadrangular shape, the center for the inside radius of the magnet and the center for the radius of a corner curvature differ in position. Thus, difficulty is encountered in designing the shape of the yoke and the shape of the magnet to be press-fitted into the yoke and in designing dies or molds for forming such shapes. In order to prevent deterioration in magnetic flux of the motor, close contact must be established between the inside of the yoke and the corner portions of the magnet, particularly, at the apexes of magnetic poles of the magnet at which the magnetic field intensity must be increased. However, when, as shown in FIG. 9(A), the center for the inside radius of the magnet and the center for the radius of a corner curvature differ in position, different references for tolerance are involved for product dimensions. Thus, difficulty is encountered in establishing close contact between the magnet and the motor casing.
Also, desirably, the magnet is reduced in weight for reduction of cost without involvement of deterioration in effective magnetic flux. Further, cogging torque must be lowered. However, the magnet having a complicated shape as shown in FIG. 9(A) encounters difficulty in designing its optimum shape for reducing weight and lowering cogging torque while increasing effective magnetic flux.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2005-20914
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. H7-59322
[Patent Document 3] Japanese Utility Model Application Laid-Open (kokai) No. S64-12455
[Patent Document 4] Japanese Patent Application Laid-Open (kokai) No. 2007-6688